The Húsavík-Flatey Fault (HFF), which is a part of the Tjörnes Fracture Zone (TFZ) in North Icela... more The Húsavík-Flatey Fault (HFF), which is a part of the Tjörnes Fracture Zone (TFZ) in North Iceland, is a ~100 km-long dextral strike-slip fault. The fault has produced many magnitude 6-7 earthquakes in the past; however, the historical earthquake record in the region is mostly limited to the past 300 years. Almost 80% of the HFF is offshore. At the on-land part of the fault, glacial morphology and lack of well-developed fluvial systems limit the number of sites having faulting-related sedimentation, which is required for conventional paleoseismic trenching. Hence, Botnsvatn Lake, located in a small pull-apart basin close to the south-eastern end of the HFF, is a valuable site to study past earthquakes in the region. Several piston cores, maximum 4.1 m-long, were collected from the lake in 2013. The physical and geochemical properties of the sediments were investigated by means of magnetic susceptibility, gamma-ray density, total carbon/nitrogen/sulphur content, grain-size distribution analysis, radiographic images on u-channels and micro-XRF scanning. The chronology of the sediments was constructed by four radiocarbon dates and 62 tephra layers. The well-documented tephrochronology in Iceland provides a robust age control along the cores. Eight sedimentary events have been detected in the sedimentary sequence of Botnsvatn Lake. Some of the events can be attributed to increased sediment influx, probably due to seismically-triggered landslides in the catchment, while the other events appear as soft sediment deformations. The youngest event in the sequence temporally coincides with the historical 1872 AD earthquake (M=6.5) in the region. In the light of preliminary interpretations, "at least" eight earthquakes took place during the last 2400 years. In the coming months, the Botnsvatn sedimentary record will be compared in detail with the paleoclimatological records in the region in order to disentangle the climatic and seismic signals
The 1963–1967 shallow-to-emergent eruption in Iceland’s Vestmannaeyjar earned a place in the hist... more The 1963–1967 shallow-to-emergent eruption in Iceland’s Vestmannaeyjar earned a place in the history of volcanology by creating the island of Surtsey while under close observation of volcanologist Sigurdur Thorarinsson (Sigurður Þórarinsson in Icelandic). This is an example of what is now called Surtseyan volcanism, and it included explosive and effusive phases from multiple vents that formed the island of Surtsey itself, as well as one fully subaqueous pyroclastic edifice and two additional, but ephemeral, islands. Sigurdur Thorarinsson identified tephra jetting and continuous uprush as characteristic types of subaerial explosive activity of Surtseyan volcanism. Subaerial cone-forming deposits of Surtseyan volcanism are typically poorly sorted, with fine-grained beds rich in sideromelane ash fragments, punctuated by larger, ubiquitously composite bombs, whereas deposits sampled by coring deep into the submarine edifice include fines-poor horizons dominated by vesicular coarse sider...
The interaction of magma with external water commonly enhances magma fragmentation through the co... more The interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine‐grained volcanic tephra. Magma‐water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water‐to‐melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench‐granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water‐to‐melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water‐to‐melt ratio is an important parameter that controls the ascent of plume in the atmosphere.
The Húsavík-Flatey Fault (HFF), which is a part of the Tjörnes Fracture Zone (TFZ) in North Icela... more The Húsavík-Flatey Fault (HFF), which is a part of the Tjörnes Fracture Zone (TFZ) in North Iceland, is a ~100 km-long dextral strike-slip fault. The fault has produced many magnitude 6-7 earthquakes in the past; however, the historical earthquake record in the region is mostly limited to the past 300 years. Almost 80% of the HFF is offshore. At the on-land part of the fault, glacial morphology and lack of well-developed fluvial systems limit the number of sites having faulting-related sedimentation, which is required for conventional paleoseismic trenching. Hence, Botnsvatn Lake, located in a small pull-apart basin close to the south-eastern end of the HFF, is a valuable site to study past earthquakes in the region. Several piston cores, maximum 4.1 m-long, were collected from the lake in 2013. The physical and geochemical properties of the sediments were investigated by means of magnetic susceptibility, gamma-ray density, total carbon/nitrogen/sulphur content, grain-size distribution analysis, radiographic images on u-channels and micro-XRF scanning. The chronology of the sediments was constructed by four radiocarbon dates and 62 tephra layers. The well-documented tephrochronology in Iceland provides a robust age control along the cores. Eight sedimentary events have been detected in the sedimentary sequence of Botnsvatn Lake. Some of the events can be attributed to increased sediment influx, probably due to seismically-triggered landslides in the catchment, while the other events appear as soft sediment deformations. The youngest event in the sequence temporally coincides with the historical 1872 AD earthquake (M=6.5) in the region. In the light of preliminary interpretations, "at least" eight earthquakes took place during the last 2400 years. In the coming months, the Botnsvatn sedimentary record will be compared in detail with the paleoclimatological records in the region in order to disentangle the climatic and seismic signals
The 1963–1967 shallow-to-emergent eruption in Iceland’s Vestmannaeyjar earned a place in the hist... more The 1963–1967 shallow-to-emergent eruption in Iceland’s Vestmannaeyjar earned a place in the history of volcanology by creating the island of Surtsey while under close observation of volcanologist Sigurdur Thorarinsson (Sigurður Þórarinsson in Icelandic). This is an example of what is now called Surtseyan volcanism, and it included explosive and effusive phases from multiple vents that formed the island of Surtsey itself, as well as one fully subaqueous pyroclastic edifice and two additional, but ephemeral, islands. Sigurdur Thorarinsson identified tephra jetting and continuous uprush as characteristic types of subaerial explosive activity of Surtseyan volcanism. Subaerial cone-forming deposits of Surtseyan volcanism are typically poorly sorted, with fine-grained beds rich in sideromelane ash fragments, punctuated by larger, ubiquitously composite bombs, whereas deposits sampled by coring deep into the submarine edifice include fines-poor horizons dominated by vesicular coarse sider...
The interaction of magma with external water commonly enhances magma fragmentation through the co... more The interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine‐grained volcanic tephra. Magma‐water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water‐to‐melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench‐granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water‐to‐melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water‐to‐melt ratio is an important parameter that controls the ascent of plume in the atmosphere.
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